The present invention relates to a solid electrolytic capacitor in which an electroconductive polymer is used as a solid electrolyte and a method for manufacturing the solid electrolytic capacitor.
In recent years, with the development of electronic devices smaller in size and capable of operating at higher frequencies, solid electrolytic capacitors using an electroconductive polymer as a solid electrolyte capable of realizing a high-frequency and low-impedance characteristics have been introduced to the market as a capacitor for use in such electronic devices. Because of use of an electroconductive polymer of a high conductivity as a solid electrolyte, this kind of solid electrolytic capacitor has an equivalent series resistance (ESR) component smaller than that of conventional dry electrolytic capacitors using an electrolytic solution or solid electrolytic capacitors using manganese dioxide, and can therefore be realized as a solid electrolytic capacitor large in capacity, small in size and close to an ideal. Therefore this kind of solid electrolytic capacitor has undergone many improvements and a number of its variations have become available on the market.
FIG. 9 is a cross-sectional view of the structure of a conventional solid electrolytic capacitor of this kind, FIG. 10 is a partially cut-away perspective view of a capacitor element constituting the electrolytic capacitor, and FIG. 11 is a perspective view of the capacitor element in a single state connected to lead frames.
The capacitor element indicated by 1 in the figures is constituted by an anode member 2 and a cathode member 3. The anode member 2 is made of aluminum and has an insulating dielectric oxide film layer 1b formed on its external surface, and an insulating layer formed of polyimide adhesive tape 4 in a predetermined position on its electrode body 1a. The cathode member 3 is constituted by cathode layers: a solid electrolyte layer 1c formed of an electroconductive polymer, a carbon paint layer 1d, an electroconductive silver paint layer 1e, formed one on another.
The anode member 2 of the capacitor element 1 is joined to an anode lead frame 11 by a certain means, e.g., welding. The cathode member 3 of the capacitor element 1 is joined to a cathode lead frame 12 by a certain means, e.g., an electroconductive adhesive. The anode lead frame 11 has a joint member 11a provided at its end and bent so as to wrap round the anode member 2 at the joint end, thereby clamping the end of the anode member 2. By welded portions 11b in the joint member 11a, the end of the anode member 2 is fixed to the joint member 11a. Guide members 12a are provided on the cathode lead frame 12 to guide the cathode member 3. An insulating casing resin 13 covers the above-described capacitor element 1 with the anode lead frame 11 and the cathode lead frame 12 partially exposed.
Various electroconductive polymers have been developed which can be used as the solid electrolyte 1c in the conventional solid electrolytic capacitor constructed as described above. The development of applications of such polymers to solid electrolytic capacitors is being promoted.
However, it is known that any of electroconductive polymers usable as the solid electrolyte 1c degrades in an oxidizing atmosphere since it is an organic material. The casing resin 13 has a role to prevent oxidation of such solid electrolyte 1c. However, air enters through a small gap between contact surfaces of the cathode lead frame 12 and the casing resin 13 to cause a reduction in conductivity, a deterioration of adhesion to the dielectric oxide film, and a reduction in stability. It is known that, from this cause, deteriorations in capacitor characteristics (a reduction in capacity and an increase in equivalent series resistance in particular) are caused particularly under a high-humidity condition.
For the purpose of solving the above-described problem in conventional solid electrolytic capacitors of this kind, the surface of the cathode lead frame 12 in contact with the casing resin 13 in which the capacitor element 1 and a part of the cathode lead frame 12 are molded is roughened to improve the adhesion between the casing resin 13 molding a part of the cathode lead frame 12 and the cathode lead frame 12. Further, a trial has been made to prevent oxidation under an oxidizing atmosphere in such a manner that, as shown in FIG. 9, the distance B between the outer end of the casing resin 13 and the end of the capacitor element 1 on the cathode side is increased by increasing the thickness of the casing resin 13 on the cathode lead frame 12 side to maximize the distance through which the cathode lead frame 12 and the casing resin 13 contact each other.
The above-described conventional solid electrolytic capacitor, however, has an increased external size because of use of a method for increasing a distance through which the cathode lead frame 12 and the casing resin 13 contact each other by increasing the thickness of the casing resin 13 so that the distance B between the outer end of the casing resin 13 and the end of the capacitor element 1 increases. Therefore there is a problem that it is extremely difficult for the conventional solid electrolytic capacitor to be reduced in size. Because of this problem, it is not possible to meet the recent strict size reduction requirement on electronic devices such as portable telephones.
In a type of solid electrolytic capacitor in which a plurality of capacitor elements 1 are stacked on anode lead frame 11 and cathode lead frame, a positioning error occurs between the stacked capacitor elements and it is necessary to determine the size of casing resin 13 by factoring in such a positioning error. This means a further increase in the degree of difficulty in the size of the solid electrolytic capacitor.
In view of the above-described problem, an object of the present invention is to provide a solid electrolytic capacitor capable of limiting oxidation deterioration of a solid electrolyte under an oxidizing condition by reducing the probability of external oxygen reaching the capacitor element and also capable of being reduced in size by reducing the thickness of the casing resin, and a method for manufacturing the solid electrolytic capacitor.
To achieve the above-described object, according to the present invention, there is provided a solid electrolytic capacitor in which an anode member and a cathode member provided in a capacitor element are respectively connected to an anode lead frame and a cathode lead frame, and in which the capacitor element including parts of the anode lead frame and the cathode lead frame are covered with a casing resin, the capacitor having a stepped portion formed in the portion of the cathode lead frame covered with the casing resin, and a gap provided between a vertical part of the stepped portion and the end of the capacitor element on the cathode member side.
In the above-described construction, the distance through which external oxygen (oxygen in atmospheric air) entering the capacitor through the gap between the cathode lead frame and the casing resin moves to reach the capacitor element can be increased to reduce the probability of external oxygen reaching the capacitor element to an extremely small value. That is, intrusion of external oxygen can be limited to ensure that oxidation deterioration does not occur easily even in an oxidizing atmosphere or at a high temperature, and that stable capacitor characteristics can be obtained under such a condition.
Therefore there is no need to maximize the distance between the outer end of the casing resin and the end of the capacitor element on the cathode member side by increasing the thickness of the casing resin as in the conventional art. Consequently, the capacitor can easily be reduced in size.
In the solid electrolytic capacitor of the present invention, a fixing portion against which an end surface of the capacitor element on the cathode member side abuts is provided inside the vertical part of the stepped portion of the cathode lead frame.
In the above-described construction, the distance through which external oxygen moves to reach the capacitor element can be increased by the distance of the fixing portion, thereby reducing the probability of external oxygen reaching the capacitor element to an extremely small value and ensuring that the oxidation deterioration does not occur easily. Further, the capacitor element can be positioned with reliability by bringing its end on the cathode member side into abutment against the fixing portion to prevent occurrence of a positioning error, thereby effectively improving the assembly accuracy. Moreover, since there is no need to factor in a positioning error in designing, the size of the capacitor can be further reduced.
In the solid electrolytic capacitor of the present invention, the fixing portion may be formed integrally with the cathode lead frame by bending and raising part of the cathode lead frame. In this manner, each of the number of component parts and the number of assembly steps can be reduced to achieve a reduction in manufacturing cost. Also, the assembly accuracy is improved by integrally forming the fixing portion to stabilize the quality.
In the solid electrolytic capacitor of the present invention, the fixing portion may be formed separately from the cathode lead frame and may be formed from the same material as that of the cathode lead frame. If the fixing portion is formed in this manner, the affinity between contact surfaces of the fixing member and the cathode lead frame or between contact surfaces of the fixing member and the casing resin is improved to stabilize the strength of connection or joint therebetween.
In the solid electrolytic capacitor of the present invention, the vertical part of the stepped portion and the end of the capacitor element on the cathode member side may be spaced apart from each other by interposing a resin constituting the casing resin between the vertical part of the stepped portion and the end of the capacitor element. The need for the fixing portion or fixing member for positioning by abutment of the end of the cathode member of the capacitor element is thereby eliminated to simplify the structure and reduce the manufacturing cost.
In the solid electrolytic capacitor of the present invention, the cathode lead frame may be bent a certain number of times to be formed into a stepped shape having a plurality of steps. This arrangement ensures that the distance through which external oxygen moves to reach the capacitor element is further increased to reduce the probability of external oxygen reaching the capacitor element to an extremely small value and to ensure that oxidation deterioration does not occur easily.
In the solid electrolytic capacitor of the present invention, the capacitor element may be a stack of a plurality of capacitor elements. This arrangement ensures that a small-size large-capacity solid electrolytic capacitor can be assembled with accuracy.
According to the present invention, there is provided a method for manufacturing a solid electrolytic capacitor including a step of connecting an anode member provided on a capacitor element by placing the anode member on an anode lead frame, a step of connecting a cathode member of the capacitor element by placing the cathode member on a cathode lead frame so that an end surface of the capacitor element on the cathode member side abuts against a fixing portion standing upright on the cathode lead frame, and a step of covering the capacitor element including parts of the anode lead frame and the cathode lead frame with a casing resin. This method makes it possible to produce with stability a small-size large-capacity solid electrolytic capacitor in which oxygen deterioration does not occur easily, and which is capable of reliable positioning of the capacitor element and has improved assembly accuracy.